In recent years, display devices such as a liquid crystal display (LCD) and an electroluminescence (EL) display are advancing in their enlargement of display screens and higher resolution as well as the higher integration of circuits by integrally forming a pixel portion and a peripheral circuit for controlling the pixel portion over a substrate.
An electroluminescence (EL) element is an element for obtaining light emission by a current flow therethrough. A display device fabricated by using the element has the advantage of wide viewing angle and high luminance since it is of a self-luminous type, which is therefore expected to be used for display devices of the next-generation.
In addition, as for an active matrix display device that integrates a pixel portion and a peripheral driver circuit over a substrate, a larger display screen and higher resolution can be obtained as opposed to a passive matrix display device, thus is supposed to be the mainstream in future.
FIG. 4A illustrates a basic structure of an active matrix EL display device. A pixel portion 402 is provided over a substrate 401. A source signal line driver circuit 403 and a gate signal line driver circuit 404 are provided around the pixel portion 402. Signal input to the source signal line driver circuit 403 and the gate signal line driver circuit 404, a current supply to EL elements and the like are carried out through a flexible print circuit (FPC) 405 from outside.
The pixel portion 402 comprises multiple pixels 411 arranged in matrix as shown in FIG. 4B, each of which light emission state is controlled to display images. Each of the pixels comprises a switching TFT 415 and a driving TFT 416, and controlled by signals from a source signal line 412 and a gate signal line 413. When the switching TFT 415 is turned ON and a video signal is inputted to the gate electrode of the driving TFT 416, a current accordingly is supplied from a current supply line 414 to an EL element 417 through the driving TFT 416, whereby light emission is obtained.
In the active matrix EL display device, luminance thereof varies according to the current value supplied to an EL element. There is a method for utilizing this for expressing gray scales, however, since TFTs are likely to have variations in the threshold values or mobility on the display screen in manufacture, there may be a case where luminance variations are caused on the display screen even with the same gray scale signal. Hereupon, there is known a digital time gray scale method by which a driving TFT is controlled to be only in two states of ON/OFF, and a gray scale is expressed by controlling the time for supplying a current to an EL element. The digital time gray scale method is described in detail in Japanese Patent Laid-Open No. 2001-343933.
In general, a current to the EL element 417 included in each pixel is supplied from outside through the FPC to a wiring provided around the display region, and then through each current supply line to each pixel as shown by an arrow in FIG. 4B. The current supply path is not necessarily like the one shown in FIG. 4B, however, the current supply path desirably has as large number of input sources as possible in general in consideration of the wiring resistance or the like.
When having a current path as shown in FIG. 4B, current is ideally supplied to the pixel portion uniformly from the current supply lines that are led out to both the upper side and the lower side. However, in practice, the amount of current flowing through a path A that is closer to the FPC is far larger than the amount of current flowing through a path B, which causes a gradient on the display screen downwardly and further from the left and right edges toward the center due to a voltage drop. When it is shown schematically, the gradient as shown in FIG. 5A is caused. The voltage drop caused on the lower side is particularly large although there is a current supply path provided by the lead wiring on the periphery.
FIG. 5B illustrates a schematic configuration diagram of a pixel 500 comprising a driving TFT, an EL element and a current supply line. As an example, a driving TFT 502 is assumed to be a P-channel TFT. Luminance control of the EL element is determined by the gate-source voltage VGS and the source-drain voltage VDS of the driving TFT 502 as shown in FIG. 5B. That is, in the graphs shown in FIG. 5C, a point A represents an operating point, and the voltage between the potential VANODE of the current supply line and the potential VCATHODE of a counter electrode is divided by the VDS of the driving TFT 502 and the Anode-Cathode voltage VEL of the EL element.
Whether the driving TFT 502 operates in a saturation region or a linear region determines each of the driving conditions.
As shown in FIG. 5C <i>, when the operating point is determined so that the driving TFT 502 operates in a saturation region, change in the current value in the operating point is small even when the EL element 503 degrades and the V-I characteristics thereof change from the solid line to the dotted line, thus change in luminance is also small. That is, a margin can be secured for the degradation of the EL element 503. Further, even when the margin causes a voltage drop on the counter electrode 504 side to a certain level, the current value does not change specifically until the transition of the operating region of the driving TFT 502 from a saturation region to a linear region. Therefore, change in luminance can be suppressed. On the other hand, since the VDS of the driving TFT 502 is increased, the drive voltage (Anode-Cathode voltage) as a whole is increased correspondingly, leading to the adversely increased power consumption.
Meanwhile, as shown in FIG. 5C <ii>, when the operating point is determined so that the driving TFT 502 operates in a linear region, the VDS of the driving TFT 502 becomes far smaller, thus the drive voltage (Anode-Cathode voltage) as a whole can be suppressed. Further, slight change in the VGS of the driving TFT 502 does not affect the image quality easily. However, as the former, the degradation of the EL element 503 directly affects the change in luminance.
Now the case is considered where the aforementioned voltage drop is caused in the current supply line 501 or the counter electrode 504. A voltage drop on the current supply line 501 side affects the source potential of the driving TFT 502. That is, the source potentials of the driving TFTs 502 have variations between the upper portion and the lower portion of the display screen, leading to the variations in the VGS. Specifically, the VGS of the driving TFTs 502 in the lower portion of the display screen is smaller than that of the upper portion thereof, leading to the small current value. That is, there are the luminance variations between the upper portion and the lower portion of the display screen. This tends to appear more frequently when the driving TFT 502 operates in a saturation region.
On the other hand, when there is no change in the characteristics of the EL element 503, a voltage drop on the counter electrode 504 side affects the drain potential of the driving TFT 502. That is, the drain potentials of the driving TFTs 502 have variations between the upper portion and the lower portion of the display screen, leading to the variations in the VDS. Specifically, the VDS of the driving TFTs 502 in the lower portion of the display screen is smaller than that of the upper portion thereof, leading to the small current value. In this case also, there are the luminance variations between the upper portion and the lower portion of the display screen. This tends to appear more frequently when the driving TFT 502 operates in a linear region.
In this manner, a voltage drop on the display screen due to the wiring resistance significantly affects the display quality. Such problem tends to arise more frequently when a current value consumed on the display screen is larger. That is, the voltage drop is an unavoidable problem when taking a large display screen into account.
In view of the aforementioned problems, the invention provides a display device that can provide favorable display quality and a driving method thereof by making the voltage distribution on the display screen uniform without the need of an additional voltage compensation circuit and the like that would cause an increase in the power consumption.
Even when current paths are provided on both of the upper portion and the lower portion of the display screen, the upper path becomes dominant due to a difference between the values of the wiring resistance, which makes it impossible to obtain an ideal voltage gradient as described above.